Federer lost his number one ranking to Rafael Nadal earlier this year
Roger Federer has withdrawn from next week's Stockholm Open, saying he is in need of a rest after a tough year.
The 27-year-old world number two was diagnosed with glandular fever at the start of the year and admitted he has been playing catch up ever since.
He also said he was unsure if he would play again before the end of the year.
"I feel fortunate to be healthy again," he said. "But I want to remain at the top of the game for years to come and go after the number one ranking again."
He added: "In order to do that, I need to get a proper rest and get strong again so that I am 100% fit for the remainder of the year or next year.
"At this point, I am not sure when I will be ready to play again, but I hope to be back at some point before the end of the year."
The former world number one was replaced at the top of the world rankings by Rafael Nadal earlier this year.
The Spaniard beat his Swiss rival in the final of the French Open and also denied Federer a sixth consecutive Wimbledon title with victory in the final at the All England Club in July.
Federer is due to play in the Madrid Masters later this month, followed by the Swiss Indoors in Basel, the Paris Masters and Tennis Masters Cup.